1. Illegal and unreported economic activity: While goods such as illegal drugs, gambling, and prostitution are sold in markets, the transactions are hidden for obvious reasons.
2. Home production and bartered goods/services: If cash doesn't change hands, the transaction will not be included in GDP. One of the somewhat misleading aspects of GDP is that whether certain things are included depends not on the nature of the good or service, but whether it was (openly) exchanged for cash.